Achieving single mode emission by introducing perturbations at prescribed positions along the length of a device is known, see EP 1 214 763 (Trinity College Dublin) the contents of which are incorporated herein by reference. So called “slotted lasers”, which achieve single longitudinal mode emission by means of optical feedback resulting from the etching of slot features along the laser cavity, are also disclosed in Irish Patent No S82521 (National University of Ireland, Cork).
In general terms, the perturbations may be caused by any index altering means which modifies the refractive index profile of the waveguide to an appropriate degree to manipulate optical feedback and hence the spectral content of the device. While the description of the present invention which follows refers primarily to the case where the perturbations are defined by slots etched along the device it will be appreciated by a person skilled in the art that the teaching of the invention is equally applicable to other forms of perturbations (for example modifying the refractive index profile by employing doping or ion implantation methods).
The term ‘slot length’ (designated Lalot in FIG. 1) as used herein refers to the distance between the longitudinal slot faces in the device material, ie ‘slot length’ is measured along the direction of light propagation, d. FIGS. 1 and 2 illustrate a typical prior art slotted laser 1 having a single rectangular slot 6. Typically such a device comprises waveguiding layers 2 (containing for example a multiple quantum well structure) covered by an upper cladding layer 4. Primary optical feedback means are provided in the form of a cleaved facet 8 at either end of the device. The distance between the facets determines the exact wavelengh's of the Fabry Perot modes of the cavity. The upper cladding layer 4 forms a ridge 3 having a cap layer 5. The slot features in such known devices are formed by etching a rectangular slot 6 in the ridge waveguide 3, resulting in two longitudinal interfaces 7 that are perpendicular to the direction of light propagation, d, within the device.
The mechanism where by slotted lasers achieve their single mode performance may be described as follows:
It is well known that the free spectral range of a laser is given by
                              Δ          ⁢                                          ⁢          λ                =                              λ            2                                2            ⁢                          n              eff                        ⁢            L                                              (        1        )            and as such is in effect determined by the cavity length, L, of the device. Where, Δλ, is the free spectral range, λ, is the free space wavelength of the light and, neff, is the effective index of the optical mode in the laser cavity. However it is observed that by placing reflective interfaces in the laser cavity at intervals separated L/N it is possible to discriminate against all but every Nth Fabry Perot mode (ie to enhance approximately every Nth mode). Where L is again the cavity length of the laser and N is an integer. This is essentially what occurs in a slotted laser, except for the fact that when a rectangular slot feature is etched into a laser cavity, two reflective interfaces are created simultaneously. What is important to note here is that each of the reflective interfaces created provides a similar amount of optical feedback. It is also important to realise that the length of the etched slot features must be kept reasonably small (typically <3 μm). The principal reasons for this are the following: Firstly, the internal loss in the waveguide beneath slots is substantially higher that elsewhere in the cavity. Secondly, since the dopant concentration in the semiconductor material below the bottom of a slot may be less than one tenth of that in the cap layer it is impossible to create a low resistance metal contact on this material. This means that if the length of a slot feature is increased arbitrarily, then a portion of material beneath the slot will remain unpumped.
In order to accurately specify the emission wavelength of a device it is necessary to be able to position all the edges of the slot features relative to each other with an accuracy that is inversely proportional to the distance between them. This can be understood by recognising that the standing wave conditions in a long cavity device are less effected by a fixed change in the length of the cavity, Δx, than the standing wave conditions in a short cavity device. (It is noted that since the facets of a device provide a significant amount of optical feedback, the positioning of these interfaces with respect to the slot features is important). As typical slotted lasers incorporate etched features, the length's of which are less than an order of magnitude greater than the wavelength of the optical field in the laser cavity. Also given that the two interfaces of a given conventional slot feature provide a significant amount of optical feedback, then it can be appreciated that the emission wavelengths, or more precisely the mirror loss spectra of such devices, are extremely sensitive to errors in the distance between the interfaces of such a feature. The emission wavelength of a slotted laser is thus critically dependent on length of the slot features themselves. The process of accurately realising a slot feature of a given length is therefore also important.
The most important factor in determining the accuracy with which a slot feature can be implemented is the choice of lithographic technique used. This varies between +/−10-20 nm for e-beam systems to +/−100-200 nm optical lithography systems. Beyond the accuracy of the lithographic system itself, the procedure of realising a rectangular slot feature of a certain length is also severely hampered by the bias associated with etching process (the offset due to process bias is designated Opb in FIG. 3). This is a problem because the length of a slot feature with parallel edges is affected by the bias of the etching process and therefore the critical dimensions in the slot pattern may be changed. As a result of these factors it is difficult, using standard lithographic and processing techniques, to sufficiently control the length of a rectangular slot feature and thus specify the spectral content of a device containing such features.
As discussed above there are considerable difficulties in accurately specifying the emission wavelength of slotted lasers. It is an object of the present invention to address these difficulties.
It is a further object of the invention to provide manufacturing method, which addresses the problems, associated with processing bias and the resulting effect on slot positioning.
It is a still further object to provide a substantially single mode laser whose performance is less temperature dependent.
It is another object of the invention to provide a method of enhancing the free spectral range of a laser and to provide a laser having improved free spectral range.